THE NERVOUS SYSTEM OF MAN 1 63 



Structural unit, it becomes a functional unit only 

 when united in what is termed the reflex arc. 

 This name was given to the relations between 

 , nerve cells before modern neurology had traced 

 out the intricate facts. It was at first thought 

 that the sensory message was reflected over onto 

 the motor nerve cells but it is now known that the 

 process is much more complicated. In all of the 

 higher animals, the possible connections between 

 the incoming sensory stimulus and the association- 

 al cells and later with the motor cells are so nu- 

 merous that one or two pathways must come to 

 dominate, if any response is to eventuate. The 

 term still has some value as a descriptive term, 

 if one is careful not to infer that by using the ex< 

 pression reflex arc, he has really explained what 

 happens. 



Before defining the synapse, the second struc- 

 ture which all animals with a nervous system have 

 in common, let us examine some of the implica- 

 tions which the phylogenetic history and the struc- 

 ture of a nerve indicate. It is a strange fact that 

 the most specialized region of the brain must gain 

 its entire information concerning the material uni- 

 verse through the so-called "fish brain" with its 

 three main sets of sense organs which are assisted 

 by the senses of taste and touch. The nerve cells 

 in the cerebrum of man receive their information 

 of the material world and cause the body to react 



